Downlink carrier aggregation is a method to increase the throughput in a cellular communications system by configuring the receiver with multiple component carriers, over which data may be received simultaneously. To facilitate carrier aggregation, the receiver needs to send Uplink Control Information (UCI) to the base station. This may comprise Hybrid Automatic Repeat request (HARQ) Acknowledgement (HARQ-ACK) bits in response to received Transport Blocks (TBs), periodic/aperiodic Channel State Information (CSI) reports and Scheduling Request (SR). Generally, the more carriers that are aggregated, the more UCI is needed. The reliability of the UCI is critical to both the system efficiency and the coverage and often comes with certain performance requirements. For example, there are stringent performance requirements for the ACK-to-NACK and NACK-to-ACK error probabilities which need to be fulfilled. It is therefore important that UCI can be transmitted reliably
In Long Term Evolution (LTE) Rel-13, carrier aggregation has been enhanced to allow aggregation of up to 32 downlink component carriers, which is in contrast to the maximum of 5 component carriers being supported so far. This not only increases the associated maximum UCI payload but also induces larger variation of the UCI payload. For example, only a few HARQ-ACK bits need to be fed back when there are few carriers aggregated. Up to two TBs may be transmitted in a subframe on a downlink component carrier and each TB is associated with one HARQ-ACK bit. Thus, the minimum HARQ-ACK payload could be 2 bits, corresponding to 2 aggregated Frequency Division Duplex (FDD) component carriers carrying 1 TB each. On the other hand, the maximum HARQ-ACK payload could theoretically reach as high as 638 bits for Time Division Duplex (TDD) with aggregation of 32 component carriers. Also the CSI payloads could vary from a few bits to several hundred of bits. Consequently, it is preferable to have a channel coding scheme for the UCI which gives reasonable performance for a variety of UCI payload sizes while allowing simple encoder and decoder structures.
For small UCI payloads, the 3rd Generation Partnership Project (3GPP) LTE-Advanced system is using various types of block codes, e.g., Reed-Muller codes are used for up to 23 HARQ-ACK/SR bits. However, these codes cannot easily be generalized to the potential large payloads that come with aggregation of up to 32 carriers.